The present invention relates to a protective lining for pressure equipment which can be used in processes for the synthesis of urea.
More specifically, the present invention relates to a lining for equipment suitable for tolerating pressures of up to 100 MPa, capable of providing adequate protection of the relative pressure-resistant body, normally made of carbon steel, from the aggressive action of typical process fluids in industrial plants for the production of urea, particularly with reference to equipment included in the synthesis cycle.
The construction technique of high pressure chemical equipment, whether it be reactors, separators, boilers, etc., normally comprises the preparation of a compact body capable of tolerating the operating pressures, guaranteeing maximum safety and time duration of the mechanical specifications, equipped with necessary passages for external communication and the inlet and outlet of process fluids. The most widely used material for this construction is steel, owing to its excellent combination of high mechanical properties, its relatively low cost and commercial availability.
Processes for the production of urea normally used in industry comprise at least one section which operates at high temperatures and pressures (synthesis loop), at which the process fluids, i.e. water, ammonia and especially saline solutions, become particularly aggressive. It has long been known that normal carbon steel is not capable of resisting the corrosion of these fluids at a high temperature and when in contact with them, undergoes a progressive deterioration which weakens the structure causing external losses and even explosions.
In these processes, ammonia, generally in excess, and carbon dioxide are reacted in one or more reactors, at pressures normally ranging from 10 to 30 MPa and temperatures between 150 and 240xc2x0 C., obtaining an aqueous solution containing urea, the non-transformed ammonium carbamate residue and the excess ammonia used in the synthesis. This aqueous solution is purified of the ammonium carbamate contained therein by its decomposition in decomposers operating, in succession, at gradually decreasing pressures. In most of the existing processes, the first of these decomposers operates at pressures which are substantially equal to the synthesis pressure or slightly lower, and basically consists of an evaporator-decomposer (more widely known as xe2x80x9cstripperxe2x80x9d, used hereafter) in which the aqueous solution of urea is heated with external vapor in the presence of a vapor phase in countercurrent which favours the decomposition of the carbamate and at the same time acts as entrainment fluid of the decomposition products. Stripping agents can be inert gases, or ammonia or carbon dioxide, or mixtures of inert gases with ammonia and/or carbon dioxide; the stripping can also possibly be carried out by using the excess ammonia dissolved in the mixture coming from the reactor (autostripping), consequently without introducing another external agent.
The decomposition products of ammonium carbamate (NH3 and CO2), together with the possible stripping agents, inert gases included, are normally condensed in a suitable condenser obtaining a liquid mixture comprising water, ammonia and ammonium carbamate, which is recycled to the synthesis reactor. In technologically more advanced plants, this condensation step is carried out at pressures substantially equal to those of the reactor or slightly lower.
As reference, among the many existing patents, U.S. Pat. Nos. 3,886,210, 4,314,077, 4,137,262 and published European patent application 504,966, can be mentioned, which describe processes for the production of urea with the above characteristics. A wide range of processes mainly used for the production of urea is provided in xe2x80x9cEncyclopedia of Chemical Technologyxe2x80x9d, 3rd Edition (1983), Vol. 23, pages 548-574, John Wiley and Sons Ed.
The most critical steps in carrying out the process are those in which the ammonium carbamate is at its highest concentration and highest temperature and consequently, in the processes mentioned above, these steps coincide with the equipment of the synthesis cycle, such as the reactor, the stripper and ammonium carbamate condenser, to mention the most important, all operating under analogous or similar conditions to those of the reactor. The problem to be solved in this equipment is that of corrosion and/or erosion particularly caused by contact with solutions of ammonium carbamate at the high temperatures and pressures necessary for the synthesis of urea.
This problem of corrosion has been confronted with various solutions in existing industrial plants and others have been proposed in literature. There are in fact numerous metals and alloys capable of withstanding for sufficiently long periods the potentially corrosive conditions arising inside a synthesis reactor of urea. Among these, lead, titanium, zirconium and several stainless steels such as, for example, AISI 316L (urea grade)steel. INOX 25/22/2 Cr/Ni/Mo steel, special austenitic-ferritic steels, etc. can be mentioned. For economic reasons however, equipment of the above type cannont be entirely constructed with these corrosion-resistant alloys or metals. Usually containers or columns are used, made of normal carbon steel, possibly multilayered, with a thickness varying from 40 to 350 mm, depending on the geometry and pressure to be tolerated (pressure-resistant body), whose surface in contact with the corrosive or erosive fluids is uniformly covered with an anticorrosive metal lining from 2 to 30 mm thick.
In particular, the reactor normally consists of a vertical container with an inlet of the reagents from below and discharge of the reaction mixture from above. The pressure-resistant body usually comprises a cylinder from 0.5 to 4 m in diameter made with a multilayer or solid wall technique, of which the two ends are closed by caps adequately welded to it. Inside the reactor, an anticorrosive lining is applied to all the walls subject to corrosion, which can consist of, for example, titanium, lead, zirconium, or preferably, stainless steels (urea grade) of the type mentioned above.
The subsequent carbamate stripper, especially if operating at the same pressure as the reactor, consists of a tube-bundle exchanger. Also in this case the pressure-resistant body is made of normal carbon steel, whereas titanium or urea-grade stainless steels are preferably used for the lining. In particular zones of the stripper there are conditions of extreme aggressivity of the fluids. This can be attributed to the high temperature, but also to the geometry of the equipment which does not allow a uniform distribution of the passivating agents, such as air, possibly combined with hydrogen peroxide, normally introduced in small quantities mixed with the process fluids.
Moreover, the injection of passivating air in the high pressure section of a urea plant can raise a risk of explosion, besides the advantage of improving the corrosion resistance of the linings most frequently used. In fact, most part of the oxygen introduced with the injected air is not consumed in the plant and is purged, mixed with the inert gas, usually from either the carbamate condenser or the top of the reactor. This gas stream contains also ammonia and hydrogen in such an amount as to produce an explosive mixture with the oxygen at the pressure and temperature conditions of the urea process, which may have catastrophic consequences in industry.
The gases leaving the stripper are usually recondensed in a carbamate condenser which is therefore in contact with a mixture similar to that of the decomposer (except for urea) and therefore extremely corrosive. Also in this case the internal lining preferably consists of the above special urea-grade stainless steels.
In the above equipment or plant units, the anticorrosive lining is obtained by the assembly of numerous elements having adequate resistance to corrosion, so as to form, at the end, a hermetically sealed structure at the high operating pressure. For the various junctions and weldings carried out for this purpose, it is frequently necessary to resort to particular techniques depending on the geometry and nature of the parts to be joined.
In the all of the above equipment, a certain number of xe2x80x9cweep-holesxe2x80x9d are effected to reveal any possible losses in the anticorrosive lining.
A weep-hole normally consists of a small tube of 8-15 mm in diameter made of corrosion-resistant material, which is inserted in the pressure-resistant body until it reaches the contact point between this and the corrosion-resistant alloy or metal lining. If there is a loss in the lining, owing to the high pressure, the internal fluid which is corrosive, immediately spreads to the interstitial zone between the lining and the pressure-resistant body and, if not discovered, causes rapid corrosion of the carbon steel of which the latter is made. The presence of weep-holes enables these losses to be revealed. For this purpose all the interstitial zones beneath the anticorrosion lining must communicate with at least one weep-hole. The number of weep-holes is normally from 2 to 4 for each ferrule which means, for example, that there are usually from 30 to 60 weep-holes in a reactor.
The material used for the protective lining is normally selected from metals or metal alloys capable of tolerating contact with the process fluids without undergoing corrosion or alterations for prolonged periods. Depending on the composition and thermal level (temperature) of the process fluids, the materials selected can differ greatly from each other, also taking into consideration their cost and specific chemical properties. Materials commonly used for the lining of equipment operating at high pressure in plants for the production of urea are, for example, stainless steel, titanium, zirconium, lead. xe2x80x9cUrea-gradexe2x80x9d stainless steels are particularly preferred, such as AISI 316L (urea-grade) steel, INOX 25/22/2 Cr/Ni/Mo steel, special austenite-ferrite steels, etc. owing to their relatively low cost and an operating performance which is sufficient to protect equipment for several years.
Inspite of their good performance, the duration of stainless steel linings however is limited and it would be preferable to have even more resistant steels. In addition, the formation of specific zones of preferential corrosion in particular plant equipment has been observed, making it necessary to resort to repair or substitution interventions of the lining more frequently than estimated on the basis of standard corrosion resistance tests. This occurs, for example, in the high pressure stripping section.
It would therefore be desirable to further improve the performance of the lining, especially in equipment operating under critical conditions, at the same time maintaining, for obvious reasons of convenience and availability, the use of stainless steels normally adopted for its construction.
It would be also desirable to have a lined equipment, particularly a urea stripper, of such a good corrosion resistance as to avoid any injection of passivating air in the plant, in order to not incur any danger of explosion.
The Applicant has now observed that resistance to corrosion in stainless steel linings is better along weldings effected during their assembley. At the same time, it has been found however that a welding deposit situated directly on the pressure-resistant body does not allow an efficient system of weep-holes to be effected owing to the lack of interstitial zones previously mentioned, and consequently the safety of the whole equipment is reduced.
On the other hand, the formation of an extensive welding deposit on a pre-existing anticorrosion lining in certain equipment, although allowing an effective weep-hole system to be maintained, causes deformation, and in certain cases damage, of the lining itself due to the great thermal and mechanical stress on a relatively thin plate subjected to tension.
The Applicant has now found a method which allows the corrosion resistance of linings to be improved also in the most critical points of a plant for the production of urea, at the same time maintaining a high safety margin, which consists in the preparation of a lining with double-layered plates.
A first object of the present invention therefore relates to a method for the construction of a double-layered stainless steel laminar element, comprising the following operations in succession:
i) preparation of a stainless steel plate, having a thickness ranging from 2 to 30 mm, preferably from 4 to 10 mm, and a surface of more than 0.1 m2, preferably between 0.5 and 5 m2;
ii) consolidated fixing of this plate to a metal support with a flat surface, preferably of a size equal to or greater than the plate itself;
iii) depositing of a welding deposit onto the surface of the plate, with a thickness ranging from 0.5 to 6 mm, preferably from 1 to 4 mm;
iv) removal of the double-layered laminar element thus obtained, from the support.
A second object of the present invention relates to a method for the protection from corrosion of chemical equipment in a plant for the synthesis of urea from ammonia and carbon dioxide at high pressure and temperature, which comprises placing a lining on the surface of this equipment exposed to process fluids, said lining at least partly consisting of laminar elements with two layers welded to each other, obtained according to the method described above.
Further objects of the present invention will be made evident in the following description and examples.
In step (i) of the manufacturing method of the present invention, the plate consists of a stainless steel or alloy of stainless steels, preferably of the type called xe2x80x9curea gradexe2x80x9d, such as, for example, AISI 316L steel (urea grade), INOX 25/22/2 Cr/Ni/Mo steel, special austenitic-ferritic steels, and others normally known to experts in the field. The selection of the most suitable material is left to the expert in the field, on the basis of the performances desired during operation. Typical examples of these steels are those commercially available under the following names: xe2x80x9c2 RE 69xe2x80x9d ((copyright), SANDVIK), xe2x80x9c724 Lxe2x80x9d ((copyright), AVESTA), xe2x80x9c725 LNxe2x80x9d ((copyright), AVESTA), xe2x80x9cDP 12xe2x80x9d ((copyright), SUMITOMO).
It is not critical, at this stage in the method of the present invention, for the plate to be preformed or shaped according to the geometry and arrangement of the double-layered element, once positioned in the relative equipment. This is in fact one of the advantages of the present invention, that the end-form of this element can be obtained with the known methods, even after its construction. For obvious reasons of greater simplicity and practicality, the plate is normally square-shaped or rectangular, with a surface extension greater than 0.1 m2, preferably between 0.5 and 5 m2. The scope of the present invention does not exclude however greater or smaller dimensions, when particular conditions require this. The plate more preferably has a width which is less than 1 m and up to 0.1 m, the length being selected each time according to necessity and in relation to the dimensions of the support used in carrying out step (ii).
The thickness of the plate is that normally used for the construction of a typical anticorrosive lining and is selected on the basis of criteria known to experts in the field. Thicknesses slightly less than the standard can be used owing to the contribution provided by the subsequent welding deposit to the resistance of the product. The thickness selected is normally greater than 2 mm to guarantee sufficient mechanical reliability, and less than 30 mm to facilitate the subsequent cutting and forming, as well as for obvious economic reasons. Preferred thicknesses are between 4 and 10 mm.
Plates of the above type are easily available and are produced with the usual methods of the iron and steel industry by lamination and cutting.
Step (ii) of the present manufacturing method comprises the consolidated fixing of the plate prepared according to step (i) on a suitable metal support. The term xe2x80x9cconsolidatedxe2x80x9d, as used in this context, refers to the fixing of the plate onto the support which allows a surface of the former to be put in substantial contact with the surface of the latter, so that efficient heat transmission is established during the subsequent depositing of the welding material.
The metal support normally consists of a plate of an adequate thickness, usually between 20 and 200 mm, and preferably between 40 and 100 mm, having at least one relatively smooth surface so as to allow adequate mechanical support of the above plate, and an efficient heat dissipation. It consists of a material which is preferably selected from metals or alloys which can be welded to the overlying steel plate, in particular, normal carbon steel or other ferrous alloys, thus allowing easy fixing by welding points. Other metal materials however can also be used for the purpose, such as, for example, aluminum, where it is possible to effect adequate fixing with different methods from welding, for example, by means of clamps, screws, screw threads, etc.
In the particular case of fixing by welding, this is carried out by points on the edge of the plate, preferably with a distance between adjacent points of 20 to 150 mm, depending on the geometry, dimensions and thickness of the plate. In this way an assembly between plate and support is obtained which is surprisingly sufficient to ensure the absence of significant deformations in the subsequent step (iii), even for plates of various square meters.
In a particular embodiment of the present invention, the support consists of a metal plate having at least one communicating hollow space with inlets to allow the circulation of a liquid inside the plate itself This further increases the heat dissipation in the subsequent step (iii). Preferred cooling liquids are selected from oils with a low viscosity and water.
The welding deposit which is extended on the plate according to step (iii) of the present method consists of a metal or metal alloy evidently compatible with the metal or metal alloy of the plate itself, as it must adhere and amalgamate on the surface to form a continuous structure with the minimum quantity of defects possible, which is a characteristic of a proper welding between two metals.
The method for extending the welding deposit can be any of the methods known in the art, for example, welding with arc-electrodes. xe2x80x9cT.I.G.xe2x80x9d (Tungsten Inert Gas) with wire rods, or by means of an automatic belt system. The operation can be indifferently carried out either manually or automatically (by belts), depending on the requirements of the case and dimensions and shape of the surface to be covered.
In a preferred embodiment of the present method, it is preferable to limit the thermal supply as much as possible during the extension of the welding deposit, in order to guarantee dimensional stability of the underlying metal plate and not to produce metal pick-ups between the two parts. This is achieved, for example, by limiting the power emitted by the welder so that no point of the surface of the plate opposite the welding welding surface (that leaning on the support) exceeds a temperature of 450xc2x0 C. Thermal flows ranging from 8000 to 16000 J/cm2 are advantageously used.
The metal or metal alloy used for the welding deposit is preferably a stainless steel of the type which is resistant to corrosion of the process fluids involved in the high pressure cycle of the synthesis of urea, particularly aqueous-ammonia solutions of carbamate and/or urea such as those present in the reactor at the bottom of the stripper or in the chamber of the carbamate condenser. These steels are known in the art and are commercially available. They contain, in addition to iron, other metals compatible with this and resistant to oxidation in an acid environment, such as, for example, Ni, V, Cr, W, Mo, etc. in sufficient quantities and combinations to make the resulting alloy corrosion resistant under the normal operating conditions. Typical examples of these steels are those previously mentioned for forming the stainless steel plate on which the welding deposit of the present invention is effected. Particularly preferred are urea grade stainless steels for welding, which have a particularly low content of ferrite and other elements different from those listed above, and can comprise appropriate additives, such as flows and fluxes, suitable for favouring melting and adhesion on the surface to be welded. Typical examples of these steels are those available on the market under the trade-names xe2x80x9cP6xe2x80x9d ((copyright), AVESTA), xe2x80x9cBatox F(U) Mxe2x80x9d ((copyright), SECHERON), xe2x80x9cThermanit 19/15 Hxe2x80x9d ((copyright), THYSSEN), xe2x80x9cNC 316 MFxe2x80x9d ((copyright), KOBE STEEL), xe2x80x9c16KCRxe2x80x9d ((copyright), ESAB), xe2x80x9cCITOXID B 316LMxe2x80x9d ((copyright), SIDEROTERMICA), xe2x80x9cNo. 4051xe2x80x9d ((copyright), KOBE STEEL), xe2x80x9cSiderfil 316 LMxe2x80x9d ((copyright), SIDEROTERMICA), xe2x80x9c20-16-3 L Mnxe2x80x9d ((copyright), SANDVIK) with flow xe2x80x9c12 b 316 LFT 2xe2x80x9d ((copyright), SOUDOMETAL), xe2x80x9c21.17.Exe2x80x9d ((copyright), THYSSEN) with flow xe2x80x9cRekord 13 BLFTxe2x80x9d ((copyright), SOUDOMETAL), xe2x80x9c25-22-2 L Mnxe2x80x9d ((copyright), SANDVIK) with flow xe2x80x9c12 b 316 LFT 2xe2x80x9d ((copyright), SOUDOMETAL), xe2x80x9c25-22-2 L Mnxe2x80x9d ((copyright), SANDVIK) with flow xe2x80x9c31 S ((copyright), SANDVIK), xe2x80x9cFOX EASN 25 Mxe2x80x9d ((copyright), VEW), xe2x80x9cThermanit 25/22 Hxe2x80x9d ((copyright), THYSSEN), xe2x80x9cSoudinox LFxe2x80x9d ((copyright), SOUDOMETAL), xe2x80x9cNC 310 MFxe2x80x9d ((copyright), KOBE STEEL), xe2x80x9cFILARC BM 310 Mo Lxe2x80x9d ((copyright), ESAB), xe2x80x9cGrinox 67xe2x80x9d ((copyright), GRIESHEIM), xe2x80x9cTGS 310 MFxe2x80x9d ((copyright), KOBE STEEL), xe2x80x9cFOX EASN 25 MIGxe2x80x9d ((copyright), VEW), xe2x80x9cGrinox T67xe2x80x9d ((copyright), GRIESHEIM), xe2x80x9c25-22-2 L Mnxe2x80x9d ((copyright), SANDVIK) with flow xe2x80x9c37 S (electroslag)xe2x80x9d ((copyright), SANDVIK), xe2x80x9c25-22 Hxe2x80x9d ((copyright), THYSSEN) with flow xe2x80x9cEST 122 (electroslag)xe2x80x9d ((copyright), SOUDOMETAL). The selection of the most suitable welding material is left to experts in the field, depending on the composition of the plate on which the welding is carried out and the final characteristics desired.
The thickness of the stainless steel plate as per step (i) is preferably uniform, even though this requisite is not essential for the purposes of the present invention. It is also preferable for the plate to be flat as this simplifies the dispersion of the heat produced by the welding deposit in step (iii) and also facilitates the fixing of the plate to the support according to step (ii). The thickness of the welding deposit deposited on the plate according to step (iii) of the present method is preferably maintained at a value which is more or less equal on the whole surface of the deposit, to guarantee uniform performance of the end-product thus obtained. In quantitative terms this thickness can have at the most a deviation from the average value of xc2x120%, preferably xc2x110%.
In the subsequent step (iv), the double-layered laminar element obtained according to the procedure of step (iii) is removed from the support onto which it was fixed using normal operations. If the fixing was effected by welding, the removal must be carried out with due precautions to avoid distorsion of the plate.
In this way, a double-layered laminar element is obtained which is essentially without deformations, and which can be used for the production of anticorrosive linings of equipment used in plants for the production of urea, comprising a first layer consisting of a stainless steel metal plate having a thickness ranging from 2 to 30 mm, preferably between 2 and 15 mm, and a surface extension of more than 0.1 m2, preferably between 0.5 and 5 m2, characterized in that the second layer has an almost uniform thickness, ranging from 0.5 to 6 mm, preferably between 1 and 4 mm, is uniformly welded to the first layer and consists of a stainless steel of the type called xe2x80x9curea gradexe2x80x9d obtained by welding deposit.
This second layer preferably consists of a welding deposit of a stainless steel selected from AISI 316L (urea grade) steels, INOX 25/22/2 Cr/Ni/Mo steels, special austenite-ferrite steels; it is more preferably obtained by the deposit of one of the particular welding materials listed above.
The present invention also relates to a method for the protection from corrosion by process fluids of equipment or elements resistant to high pressures of a plant for the production of urea, particularly in the synthesis section, comprising the production of a hermetically sealed lining of at least a part of the surface of this equipment in contact with process fluids, by means of one or more of the above double-layered laminar elements of the present invention, suitably shaped and welded to each other.
The selection of the most suitable construction technology among the many known methods for the production of the protective lining of the present invention is left to experts in the field, comprising cutting and welding methods, as well as those for obtaining weep-holes in the most appropriate points, the annealing of the weldings on the pressure-resistant body, the application of welding deposits below the welding lines, and also additional protection in the case of accidental losses, the formation of communication points or slots between the various interstitial zones beneath the lining and among these weep-holes, the shaping methods of the laminar elements, such as calendering or moulding, and all the other known techniques which can be used for the purpose.
The above method of the present invention allows the corrosion resistance of equipment involved in the synthesis process of urea to be improved, maintaining all the elements necessary for guaranteeing the safety of the plant and also enables accidental losses to be revealed. In fact, this lining is produced with the known methods used for traditional linings, i.e. by placing the double-layered elements onto the underlying pressure-resistant body without extensive welding, but only welding the edges to each other and to the underlying pressure-resistant body, thus forming interstices between lining and pressure-resistant body which communicate with each other and with a system of weep-holes to reveal any possible losses.
On the contrary, an extensive welding deposit directly on the pressure-resistant body would not make it possible to maintain an efficient safety system based on weep-holes, as there would not be interstitial spaces suitable as outlets for the corrosive fluids in the case of losses of the lining. In these cases the corrosive process fluid would not be revealed and would remain in contact with the carbon steel of the pressure-resistant body causing its corrosion and jeopardizing the structure.
According to a particular aspect of the present invention, not all the surface of the equipment is lined with the above double-layered laminar elements having improved resistance to corrosion, but optionally, only the part attributed as being the most exposed to corrosion. For example, in the case of stripping equipment, a lining can be produced with double-layered elements in the lower section where the process temperature is higher, providing a traditional type lining, evidently less expensive, in the upper section which is less exposed to corrosive attack.
As previously specified, the method of the present invention can be particularly applied to the high or medium pressure section of a synthesis plant of urea. This substantially refers to synthesis reactors of urea, equipment for the decomposition of non-transformed carbamate (particularly strippers), and containers for the condensation of NH3 and CO2 with the formation of carbamate solutions.
This equipment operates at pressures normally ranging from 10 to 50 MPa and temperatures ranging from 70 to 300xc2x0 C., in the presence of mixtures containing water, ammonia, carbon dioxide and ammonium carbamate which is the condensation product of these compounds according to the reaction:
[2NH3+CO2+nH2Oxe2x86x92NH4OCONH2xc3x97nH2O]